1. Field of the Invention
The present invention demonstrates how a new Pd/Al.sub.2 O.sub.3 catalyst impregnated with NaOH may be used for decomposing NO from combustion exhausts, and its required operational temperature may be lowered to 900.degree. K.
2. Description of the Prior Art
NO.sub.x is an atmospheric pollutant that may cause respiratory tract diseases, acid rain and damages to the ozone layer. The majority of NO produced by human civilization are from high temperature (T&gt;1500) combustion. The two main sources being the thermoelectric plants and internal-combustion engines. Under high temperature, oxygen and nitrogen in the atmosphere will generate NO, via intermediates of free radicals: EQU N.sub.2 +xO.sub.2.fwdarw.2NO.sub.x (1)
Reaction (1) is an endothermic reaction. Thus when temperature rises due to combustion, the equilibrium of the above reaction will lean toward the right-hand side and generate a large amount of NO.sub.x. This should be eliminated from exhaust gases on discharge so as to protect the ecological environment.
So far M. Shelef has proposed a method in Chem. Rev, Vol. 95, pp. 209, 1995 that eliminates NO.sub.x in the exhaust gas via the SCR method of using NH.sub.3 (NH.sub.2).sub.2, or (NH.sub.2).sub.2 CO as the reducing agent to catalyst and through the following reaction: ##EQU1##
But this method consumes reducing agents and has both the problem of controlling the dosage of reducing agent, and high operating costs.
Fortunately the reverse reaction of (1) is an exothermic reaction. The equilibrium of reaction shifts to the left-hand side at low temperature. Therefore, with appropriate catalyst, NO will decompose into Nitrogen and Oxygen: ##EQU2##
M. Iwamoto and others proposed in Catal. Today Vol.10, p. 57 (1991) that the Cu/Zeolite catalysts can obtain good NO decomposing activity at 800.degree. K. Under the condition of reaction, however, the M. Iwamoto group reported in J. Phys. Chem. Vol. 95, p. 3727 (1991) that the above material will suffer from the problem of sulfur poisoning and a losing of Al from Zeolites and thus will become deactivated. Therefore it is necessary to switch to the other catalysts.
For NO.sub.x in the exhaust gases of vehicle, it is already known that the cleaning effect can be accomplished with three-way catalysts under the condition of controlled fuel-air ratio, by CO and RH reduction: EQU RH+CO+NO.sub.x.fwdarw.CO.sub.2 +H.sub.2 O+N.sub.2 (4)
The main active ingredients of this three-way catalyst are Rh, Pt and Pd. Since Rh and Pt are more expensive, J. S. Hepburn and others already reported in SAE 941058 of a trend to replace them with mass-produced and cheaper (Pd) mono-metal catalyst. But the decomposition rate of Pd mono-metal catalyst against NO is low, and it is still necessary to improve this catalyst.
For the above reason, the inventors reported in Appl. Catal. B.: Environmental Vol. 6, pp.105 on developing Pd/Al.sub.2 O.sub.3 as catalyst to promote reaction (3). The study showed that this catalyst indeed has good catalytic activity in reaction (3) at 1100K. The degree of activity is connected to the quantity of Pd applied. The mechanism of the catalytic reaction is as follows: EQU NO+.left brkt-top.*.right brkt-bot..fwdarw..left brkt-top.*-NO.right brkt-bot. (5) EQU .left brkt-top.*-NO.right brkt-bot.+.left brkt-top.*.right brkt-bot..fwdarw..left brkt-top.*-N.right brkt-bot.+.left brkt-top.*-O.right brkt-bot. (6) EQU 2.left brkt-top.*-N.right brkt-bot..fwdarw.2.left brkt-top.*.right brkt-bot.+N.sub.2 (7) EQU 2.left brkt-top.*-O.right brkt-bot..fwdarw.2.left brkt-top.*.right brkt-bot.+O.sub.2 (8)
.left brkt-top.*.right brkt-bot. here represents Pd atoms exposed on the surface of catalyst in the above equations. Steps (5).about.(7) in this reaction mechanism can be reacted smoothly at temperature as low as 500 K, but the activation energy of step (8) is so high (&gt;200 kJ mol.sup.-1) that the reaction obtains a faster rate only at temperatures as high as T&gt;1000 K. therefore step (8) is the key step which hinders the reaction rate.
The inventors' research further showed that by improving the Pd/Al.sub.2 O.sub.3 catalyst with Au, Ag or Cu metal additives, the required activation energy of reaction (8) will be reduced for 30 kJ mol.sup.-1. Therefore the reaction temperature in reaction (3) can be reduced slightly. This research has been granted Japan patent No. 2,631,628 in 1997.
Recently the research of inventors showed that by coating the Pd/Al.sub.2 O.sub.3 catalyst with NaOH, besides enhancing the original catalytic activity of reaction (3) at 1100 K, it produced a new peak of N.sub.2 production at around 900 K. The activity of this region varies with the amount of NaOH added.
When the reaction gas (4% NO/He) passes through a 0.8 g catalytic bed at a flow rate of 30 ml ml.sup.-1, NO content of the gas will be decomposed into nitrogen and oxygen. The present invention uses a 5A molecular sieve analyzing-tube to separate the products of reaction, and used the TCD to perform vapor phase layer analysis.